The present invention relates to a system and technique for converting a signal to a digital value and more particularly to directly converting a frequency modulated sinusoidal signal to a digital output.
In modern navigation systems, rate and acceleration sensors are employed in attitude and heading reference systems. Such navigation systems, as are used in aircraft, missiles and other space vehicles, have advanced significantly with the advent of precision instruments and sophisticated computer technology. Such systems attempt to provide increased performance and reliability at a reasonable cost so that they may be economically incorporated in aircraft to provide the improved navigation benefits.
In one type of a conventional navigation system known as a strapped-down system, inertial devices are rigidly affixed and oriented with respect to a predetermined axis of a vehicle. A rotating rate and acceleration sensor may then be used to provide signal outputs which represent rate and acceleration measurements in multiple axes of the vehicle for providing information utilized in the navigation and control of the vehicle. In the past, such strapped-down systems have been costly because of the need for highly accurate mechanical assemblies and sensors and their accuracy has been limited by the signal processing capabilities used to demodulate the signals representing rate and acceleration. Although improved and less costly sensors have been developed, there are still many problems caused by unwanted harmonics, noise, phase shift and misalignment errors which prevent the accurate measurement of desired parameters.
In one example of a prior known navigation system employing a strapped-down technique, as disclosed in U.S. Pat. No. 4,444,053 entitled "Sensor Assembly for Strapped-Down Attitude and Heading Reference System" by B. F. Rider, filed Apr. 21, 1982 and assigned to the same assignee as this application, a rotating rate and acceleration sensor is disclosed which provides rate and acceleration measurement in multiple axes. Although the aforementioned system provides improved accuracy through the use of low-cost piezoelectric sensors, the nominally sinusoidal signals representing rate and acceleration must be coupled from the rotating sensor to external circuits capable of demodulating the signals to provide the rate and acceleration measurements. As disclosed in that application, the signals representing rate and acceleration are first converted from a voltage to a modulated frequency and then coupled by means of rotary transformers to external circuitry including frequency-to-voltage converters, sine/cosine demodulators and thereafter analog-to-digital converters for providing digital outputs for interfacing with appropriate digital control circuitry. While the system provides improved accuracy at less cost, the demodulation of the signal from the rotating sensor still produces inaccuracies and errors which have not been completely resolved. There is therefore a continuing need for signal demodulation systems which are capable of converting measured signals in navigation and other similar systems to digital values with improved accuracy.
The present invention has therefore been developed to overcome the shortcomings of the above known and similar techniques and to provide a signal processing system which generates a digital value directly from the frequency modulated signal produced by a sensor element.